Conventional wavelength division multiplexing (WDM) couplers, such as the one disclosed in FIG. 1, include a double bore ferrule 21 encasing the ends of optical fibers 11 and 12, which are optically coupled to a lens 31. The lens 31 collimates a beam of light from the fiber 11 onto an optical filter 41, which passes light centered around a single wavelength. This wavelength corresponds to one or more channels. The remainder of the light is reflected back through the lens 31 and out the fiber 12. The light that is passed through the filter 41 is focused by a lens 32 onto a fiber 13, which is encased in a ferrule 22. Alternatively, when used as a multiplexer, light from the fiber 12 is reflected by the filter 41 and combined with light passing therethrough from fiber 13. The combined signal is output the fiber 11.
There are certain drawbacks to these conventional couplers, such as a certain amount of cross talk between the fibers 11 and 12 due to their close proximity. The center wavelength (λp) passed by the optical filter 41 is dependent upon the angle of incidence of the light hitting the filter, i.e. λp=λ0(1−c·sin2θ)1/2 in which c is a constant dependent upon the particular filter. However, due to the fact that the position of filter 41 is fixed, the center wavelength of the passband of the filter cannot easily be tuned. As a result, selecting the appropriate filter for the desired passband before manufacturing the coupler becomes critical.
One type of optical device provided as a solution for overcoming these shortcomings is disclosed in U.S. Pat. No. 4,244,045 issued Jan. 6, 1981 to Nippon Telegraph and Telephone. During the manufacture of these devices, each optical filter can be individually angle tuned to maximize optical coupling. However, as is well known in the art, the bandwidth of the passband decreases as the angle of incidence increases, resulting in an increase in the insertion loss of the filter. Furthermore, polarization dependent loss (PDL), i.e. the difference in the transmission of the S and P components of the signal, greatly increases with an increase in the angle of incidence.
Accordingly, what is required is a device that enables the filters to be angle tuned, while minimizing the angle at which the light is incident on the filter. However, this raises another problem related to the physical restraints of positioning two ports, each with their own lens and ferrule, beside each other. With reference to FIG. 2, we will assume that for a beam of diameter Φ=0.46 mm the distance D between the ports must be at least 6.5 mm. If we also assume that the angle of incidence between the beam of light and the filter normal must be 1.8° or less, we can calculate the distances L1 and L2 to be approximately 100 mm each for a total beam path L1+L2 of 200 mm. This is a great distance for the beams to travel, which would require an unnecessarily large package to support. FIG. 3 schematically represents a multi-filter WDM platform that would be required assuming the aforementioned calculations. This device includes a plurality of filters 41, between which a WDM signal bounces while dropping channels at the ports.
An object of the present invention is to overcome the shortcomings of the prior art by providing a compact wavelength divisional multiplexer with tunable optical filters and without requiring relatively large angles of incidence.